There are various types of hydraulic steering apparatuses, which can be classified mainly into two types: a handle type and a joystick lever type.
(1) The handle type is well known and can be classified mainly into two types: a follow-up linkage type and a total hydraulic type. The handle type allows an operator to rotate a handle provided adjacent to a cab seat in order to obtain a steering angle proportional to a rotary operation angle of the handle. The maximum rotary operation angle is around two to three clockwise or counterclockwise rotations, for example.
In the follow-up linkage type, when the operator rotates the handle clockwise, a steering valve directly connected to the handle is switched from a neutral position to a right steering position. This allows pressurized oil from a hydraulic pump to flow into a hydraulic cylinder so as to operate the cylinder, via a steering valve. The hydraulic cylinder is attached to a body and tie rods at both ends, and therefore the vehicle starts a right steering operation when the hydraulic cylinder operates. Between the tie rod and the steering valve, there is supported a link mechanism (called a follow-up linkage) which attempts to return the steering valve to the neutral position when the vehicle is steered. Accordingly, if the operator stops the clockwise rotation of the handle, the steering valve returns from the right steering position to the neutral position, following the stop operation, to stop the pressurized oil supply to the hydraulic cylinder. In other words, the right steering operation is completed with a steering angle corresponding to a rotary operation angle. The left steering operation is the same as the right steering operation.
In the total hydraulic type, when the operator rotates the handle clockwise, a rotary steering valve (called an orbit roll steering valve) is switched from the neutral position to the right steering position. This allows pressurized oil from a first hydraulic pump to flow to the hydraulic cylinder, as to operate the cylinder, via the rotary steering valve and via a second hydraulic pump integrally mounted onto a rotary shaft of the rotary steering valve. The hydraulic cylinder is attached to a body at each end, and therefore the vehicle starts a right steering operation when the hydraulic cylinder is actuated. If the operator stops the clockwise rotation, the second hydraulic pump provided in the rotary steering valve automatically returns the rotary steering valve from the right steering position to the neutral position, which stops the pressurized oil supply to the hydraulic cylinder. The second hydraulic pump rotates by a rotary operation angle to supply pressurized oil of a flow rate based on the rotary operation angle to the hydraulic cylinder. Accordingly, the right steering operation is completed with a steering angle approximately equal to the rotary operation angle. The left steering operation is the same as the right steering operation.
(2) The joystick lever type is more often used in recent times. In this type, it is possible to obtain a steering speed approximately proportional to a tilt operation angle when the operator tilts a joystick lever provided adjacent to the cab seat. The maximum tilt operation angle is equal to a rotary angle of the operator's wrist tilting leftward or rightward (in other words, a minor operation angle control), and therefore it is impossible to expect a wide range of operation angles (in other words, a major operation angle control) such as two to three clockwise or counterclockwise rotations, as obtained in the handle type. Therefore, this type of the apparatus has a configuration as described below.
If the operator tilts the joystick lever rightward, the steering valve moves from the neutral position toward the right steering position by an amount approximately proportional to the tilt operation angle. In other words, openings of flow paths from the hydraulic pump to the hydraulic cylinder in the steering valve are increased or decreased in proportion to the tilt operation angle. This allows pressurized oil from the hydraulic pump to flow to the hydraulic cylinder via the steering valve by a flow rate per unit approximately proportional to the tilt operation angle, so as to actuate the hydraulic cylinder. The hydraulic cylinder is attached to a body at each end in the same manner as for the handle type. Accordingly, when the hydraulic cylinder is actuated, the vehicle starts a right steering operation. Subsequently, when the operator judges that an actual steering angle of the vehicle equals a target steering angle by visual observation or skill, or at a desired timing, he returns the joystick lever to a central position. By this operation, the steering valve returns from the right steering position to the neutral position to stop the pressurized oil supply to the hydraulic cylinder. In other words, the steering speed is approximately proportional to the tilt operation angle, and the right steering operation is completed by the return of the joystick lever to the, central position by the operator. The left steering operation is the same as the right steering operation.
The prior art set forth above, however, has the following problems:
(1) In the handle type apparatus, when the operator stops the rotary operation, the steering valve automatically returns to the neutral position. Therefore, like the joystick lever type, the operator need not return the steering valve to the neutral position each time. In addition, the handle type has an advantage that smooth control is achieved at a minute steering angle since it is possible to obtain a steering angle corresponding to a rotary operation angle, in addition to a great steering angle per unit rotary operation angle. This advantage, however, means that a great rotary operation angle is required to obtain a great steering angle. Therefore, in a wheel loader which requires a frequent switching between a right steering position and a left steering position or in a vehicle such as a forklift which requires a rotary operation of three to four rotations, for example, a complicated operation is required. In addition, an operator is required to be skilled, which causes fatigue.
(2) The joystick lever type requires the operator to return the joystick lever to the central position whenever the actual steering angle becomes equal to the target steering angle, so as to return the steering valve from the steering position to the neutral position. In other words, the number of the operations is twice that of the handle type, which causes fatigue. In addition, the maximum tilt operation angle is a rotary angle of his wrist tilting leftward or rightward, and therefore, although there is an advantage in that the operator can control the vehicle with his wrist, the steering speed becomes higher as the tilt operation angle is increased. The joystick lever type has the following problems.
An abrupt increase of the tilt operation angle causes a shock which may shake the body at a start or a stop of joystick tilting. On the other hand, if the tilt operation angle is decreased, it takes a lot of time until the actual steering angle becomes equal to the target steering angle, wherein the operator cannot obtain an operational feeling obtained by the handle type. As set forth above, the tilt operation angle is irrelevant to the steering angle, and therefore it is not preferable to use the joystick lever type in a vehicle having wheels which cannot be seen directly.
In addition, while traveling at high-speed on a road, the steering angle must be controlled minutely and frequently as compared to control while traveling at low-speed. An operator can easily set a large tilt operation angle by mistake during an emergency, causing an abrupt change in steering. Therefore, it is not preferable to adopt this type of steering apparatus directly to a vehicle which travels at high speed.
Furthermore, in a wheel loader which requires quick and sharp switching between a right steering position and a left steering position, or in a forklift which requires a great steering angle, it is inevitable that a great tilt operation angle will be set. In this case, an abrupt change in steering is also caused, which leads to operator fatigue. If the maximum opening area of the steering valve is decreased as a countermeasure, the working efficiency of the vehicle is lower than that of the handle type.